Method for operating a constant pressure filament driver to an extruder head in a three-dimensional object printer

ABSTRACT

A method of operating an additive manufacturing system feeds solid extrusion material into a heater using a slip clutch coupled to an actuator of a mechanical driver to supply thermoplastic material into a manifold in an extruder head. The method sets a speed of the actuator so the actuator operates at a rotational speed that is slightly greater than the rotational speed of the mechanical mover. This method helps maintain the pressure of the thermoplastic material in the manifold of the extruder head in a predetermined range no matter how many nozzles are opened in the extruder head.

PRIORITY CLAIM

This application is a divisional application of pending U.S. patentapplication Ser. No. 15/334,721, which is entitled “Constant PressureFilament Driver For Extruder Heads In Three-Dimensional ObjectPrinters,” which was filed on Oct. 26, 2016, and which issued as U.S.Pat. No. 10,682,796 on Jun. 16, 2020.

TECHNICAL FIELD

This disclosure is directed to extruders used in three-dimensionalobject printers and, more particularly, to extruders that are fedmaterial by an extrusion material supply system.

BACKGROUND

Three-dimensional printing, also known as additive manufacturing, is aprocess of making a three-dimensional solid object from a digital modelof virtually any shape. Many three-dimensional printing technologies usean additive process in which an additive manufacturing device formssuccessive layers of the part on top of previously deposited layers.Some of these technologies use extruder heads that soften or meltextrusion material, such as ABS plastic, into thermoplastic material andthen emit the thermoplastic material in a predetermined pattern. Theprinter typically operates the extruder head to form successive layersof the thermoplastic material that form a three-dimensional printedobject with a variety of shapes and structures. After each layer of thethree-dimensional printed object is formed, the thermoplastic materialcools and hardens to bond the layer to an underlying layer of thethree-dimensional printed object. This additive manufacturing method isdistinguishable from traditional object-forming techniques, which mostlyrely on the removal of material from a work piece by a subtractiveprocess, such as cutting or drilling.

The thermoplastic material is stored in a manifold in the extruder head.The amount of thermoplastic material emitted by the one or more nozzlesin the extruder head varies during production of an object. Thesevariations are caused by the number of nozzles in the extruder head, therate at which the extruder head and object supporting surface moverelative to one another, the area of an object being formed, thetemperature of the material, and the like. The fluctuating flow rate ofthermoplastic material in the manifold affects the pressure of thematerial for its delivery through the nozzle or nozzles. A pressurewithin a suitable range would be preferred to enable each nozzle to emita properly formed stream of the material. An extruder head thatmaintains a pressure in the manifold within a predetermined range duringextrusion of the thermoplastic material would be beneficial.

SUMMARY

A new apparatus enables the pressure of the thermoplastic materialstored in a manifold of an extruder head to be maintained within apredetermined range. The apparatus includes an extruder head having amanifold configured to store thermoplastic material and at least onenozzle through which thermoplastic material from the manifold can beemitted, a heater having a channel through which extrusion material canpass and at least one heating element configured to thermally treat theextrusion material in the channel to melt the extrusion material to formthermoplastic material, the channel in the heater being fluidlyconnected to the manifold in the extruder head to enable thethermoplastic material to enter the manifold, an actuator having anoutput shaft, a mechanical mover operatively connected to the outputshaft of the actuator to enable the actuator to operate the mechanicalmover, the mechanical mover being positioned to apply a force to theextrusion material to move the extrusion material from a supply ofextrusion material to the channel in the heater when the actuatoroperates the mechanical mover, and a slip clutch operatively connectedto the output shaft of the actuator to limit a force exerted by themechanical mover to the extrusion material.

A new method enables the pressure of the thermoplastic material storedin a manifold of an extruder head to be maintained within apredetermined range. The method includes operating a mechanical mover toexert a force within a predetermined range to move extrusion materialinto a heater, operating the heater to melt the extrusion material andform thermoplastic material that moves towards an extruder head thatextrudes the thermoplastic material, and limiting with a slip clutch aforce exerted by the mechanical mover to the extrusion material.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and other features of systems that feed extruderheads are explained in the following description, taken in connectionwith the accompanying drawings.

FIG. 1 depicts an additive manufacturing system that includes a solidextrusion material feeding system that maintains pressure ofthermoplastic material within a manifold of an extruder head within apredetermined range.

FIG. 2 is a block diagram of a feedback system implemented by thecontroller in the system of FIG. 1.

FIG. 3 is a block diagram of an alternative feedback system implementedby the controller in the system of FIG. 1.

FIG. 4 is a diagram of a prior art three-dimensional object printerhaving a multi-nozzle extrusion printhead that does not have the solidextrusion material feeding system of FIG. 1.

DETAILED DESCRIPTION

For a general understanding of the environment for the device disclosedherein as well as the details for the device, reference is made to thedrawings. In the drawings, like reference numerals designate likeelements.

As used herein, the term “extrusion material” refers to a material thatis softened or melted to form thermoplastic material to be emitted by anextruder head in an additive manufacturing system. The extrusionmaterials include, but are not strictly limited to, both “buildmaterials” that form permanent portions of the three-dimensional printedobject and “support materials” that form temporary structures to supportportions of the build material during a printing process and are thenoptionally removed after completion of the printing process. Examples ofbuild materials include, but are not limited to, acrylonitrile butadienestyrene (ABS) plastic, polylactic acid (PLA), aliphatic or semi-aromaticpolyamides (Nylon), plastics that include suspended carbon fiber orother aggregate materials, electrically conductive polymers, and anyother form of material that can be thermally treated to producethermoplastic material suitable for emission through an extruder head.Examples of support materials include, but are not limited to,high-impact polystyrene (HIPS), polyvinyl alcohol (PVA), and othermaterials capable of extrusion after being thermally treated. In someextrusion printers, the extrusion material is supplied as continuouselongated strand of material commonly known as a “filament.” Thisfilament is provided in a solid form by one or more rollers pulling theextrusion material filament from a spool or other supply and feeding thefilament into a heater that is fluidly connected to a manifold withinthe extruder head. The heater softens or melts the extrusion materialfilament to form a thermoplastic material that flows into the manifold.When a valve positioned between a nozzle and the manifold is opened, aportion of the thermoplastic material flows from the manifold throughthe nozzle and is emitted as a stream of thermoplastic material. As usedherein, the term “melt” as applied to extrusion material refers to anyelevation of temperature for the extrusion material that softens orchanges the phase of the extrusion material to enable extrusion of thethermoplastic material through one or more nozzles in a printhead duringoperation of a three-dimensional object printer. The melted extrusionmaterial is also denoted as “thermoplastic material” in this document.As those of skill in the art recognize, certain amorphous extrusionmaterials do not transition to a pure liquid state during operation ofthe printer.

As used herein, the terms “extruder head” refers to a component of aprinter that melts extrusion material in a single fluid chamber andprovides the melted extrusion material to a manifold connected to one ormore nozzles. Some extruder heads include a valve assembly that can beelectronically operated to enable thermoplastic material to flow throughnozzles selectively. The valve assembly enables the independentconnecting of one or more nozzles to the manifold to extrude thethermoplastic material. As used herein, the term “nozzle” refers to anorifice in an extruder head that is fluidly connected to the manifold inan extruder head and through which thermoplastic material is emittedtowards an image receiving surface. During operation, the nozzle canextrude a substantially continuous linear arrangement of thethermoplastic material along the process path of the extruder head. Acontroller operates the valves in the valve assembly to control whichnozzles connected to the valve assembly extrude thermoplastic material.The diameter of the nozzle affects the width of the line of extrudedthermoplastic material. Different printhead embodiments include nozzleshaving a range of orifice sizes with wider orifices producing lineshaving widths that are greater than the widths of lines produced bynarrower orifices.

As used herein, the term “manifold” refers to a cavity formed within ahousing of an extruder head that holds a supply of thermoplasticmaterial for delivery to one or more nozzles in the printhead during athree-dimensional object printing operation. A system, which isdescribed in more detail below, is configured to feed extrusion materialinto a heater at a rate that maintains a pressure on the thermoplasticmaterial in the manifold within a predetermined range. That pressureenables the rate at which the one or more nozzles extrude thethermoplastic material to be regulated. Specifically, even though thevalves connected to the multiple nozzles are activated and deactivatedon an individual basis, the filament feed system enables the manifold tosupply thermoplastic material through any activated valves to nozzles inthe extruder head at a substantially constant rate even as the number ofactivated valves changes during a printing operation.

As used herein, the term “arrangement of extrusion material” refers toany pattern of the extrusion material that the extrusion printhead formson an image receiving surface during a three-dimensional object printingoperation. Common arrangements of extrusion material includestraight-line linear arrangements of the extrusion material and curvedarrangements of the extrusion material. In some configurations, theextruder head extrudes the thermoplastic material in a continuous mannerto form the arrangement with a contiguous mass of the extrusion materialwhile in other configurations the extruder head operates in anintermittent manner to form smaller groups of thermoplastic materialthat are arranged along a linear or curved path. The three-dimensionalobject printer forms various structures using combinations of differentarrangements of the extrusion material. Additionally, a controller inthe three-dimensional object printer uses object image data and extruderhead path data that correspond to different arrangements of theextrusion material prior to operate the extruder head and form eacharrangement of the extrusion material. As described below, thecontroller optionally adjusts the operation of the valve assembly toform multiple arrangements of thermoplastic material through one or morenozzles during a three-dimensional printing operation.

As used herein, the term “process direction” refers to a direction ofrelative movement between an extruder head and an image receivingsurface that receives thermoplastic material extruded from one or morenozzles in the head. The image receiving surface is either a supportmember that holds a three-dimensional printed object or a surface of thepartially formed three-dimensional object during an additivemanufacturing process. In the illustrative embodiments described herein,one or more actuators move the extruder head about the support member,but alternative system embodiments move the support member to producethe relative motion in the process direction while the extruder headremains stationary. Some systems use a combination of both systems fordifferent axes of motion.

As used herein, the term “cross process direction” refers to an axisthat is perpendicular to the process direction in the plane of theprocess direction. The process direction and cross-process directionrefer to the relative path of movement of the extruder head and thesurface that receives the thermoplastic material. In someconfigurations, the extruder head includes an array of nozzles thatextend along the cross-process direction. Adjacent nozzles within theextruder head are separated by a predetermined distance in thecross-process direction. In some configurations the system rotates theextruder head to adjust the effect cross-process direction distance thatseparates different nozzles in the extruder head to adjust thecorresponding cross-process direction distance that separatesarrangements of the thermoplastic material that are extruded from thenozzles in the extruder head.

During operation of the additive manufacturing system, an extruder headmoves in the process direction along both straight and curved pathsrelative to a surface that receives thermoplastic material during thethree-dimensional object printing process. Additionally, an actuator inthe system optionally rotates the extruder head about the Z axis toadjust the effective cross-process distance that separates nozzles inthe extruder head to enable the extruder head to form two or morearrangements of thermoplastic material with predetermined distancesbetween each arrangement of the thermoplastic material. The extruderhead moves both along the outer perimeter to form outer walls of atwo-dimensional region in a layer of the printed object and within theperimeter to fill all or a portion of the two-dimensional region withthe thermoplastic material.

FIG. 4 depicts a prior art three-dimensional object additivemanufacturing system or printer 100 that is configured to operate anextruder head 108 to form a three-dimensional printed object 140.Although the printer 100 is depicted as a printer that uses planarmotion to form an object, other printer architectures can be used withthe extruder head and mechanical mover of extrusion material describedin this document. These architectures include delta-bots, selectivecompliance assembly robot arms (SCARAs), multi-axis printers,non-Cartesian printers, and the like. The printer 100 includes a supportmember 102, a multi-nozzle extruder head 108, extruder head support arm112, controller 128, memory 132, X/Y actuators 150, an optional Zθactuator 154, and a Z actuator 158. In the printer 100, the X/Yactuators 150 move the extruder head 108 to different locations in atwo-dimensional plane (the “X-Y plane”) along the X and Y axes toextrude arrangements of thermoplastic material that form one layer in athree-dimensional printed object, such as the object 140 that isdepicted in FIG. 4. For example, in FIG. 4 the X/Y actuators 150translate the support arm 112 and extruder head 108 along guide rails113 to move along the Y axis while the X/Y actuators 150 translate theextruder head 108 along the length of the support arm 112 to move theprinthead along the X axis. The extruded patterns include both outlinesof one or more regions in the layer and swaths of the thermoplasticmaterial that fill in the regions within the outline of thermoplasticmaterial patterns. The Z actuator 158 controls the distance between theextruder head 108 and the support member 102 along the Z axis to ensurethat the nozzles in the extruder head 108 remain at a suitable height toextrude thermoplastic material onto the object 140 as the object isformed during the printing process. The Zθ actuator 154 controls anangle of rotation of the extruder head 108 about the Z axis (referencedas Zθ in FIG. 4) for some embodiments of the extruder head 108 thatrotate about the Z axis. This movement controls the separation betweennozzles in the extruder head 108, although some extruder heads do notrequire rotation during the manufacturing process. In the system 100,the X/Y actuators 150, Zθ actuator 154, and the Z actuator 158 areembodied as electromechanical actuators, such as electric motors,stepper motors, or any other suitable electromechanical device. In theillustrative embodiment of FIG. 4, the three-dimensional object printer100 is depicted during formation of a three-dimensional printed object140 that is formed from a plurality of layers of thermoplastic material.

The support member 102 is a planar member, such as a glass plate,polymer plate, or foam surface, which supports the three-dimensionalprinted object 140 during the manufacturing process. In the embodimentof FIG. 4, the Z actuator 158 also moves the support member 102 in thedirection Z away from the extruder head 108 after application of eachlayer of thermoplastic material to ensure that the extruder head 108maintains a predetermined distance from the upper surface of the object140. The extruder head 108 includes a plurality of nozzles and eachnozzle extrudes thermoplastic material onto the surface of the supportmember 102 or a surface of a partially formed object, such the object140. In the example of FIG. 4, extrusion material is provided as afilament from extrusion material supply 110, which is a spool of ABSplastic or another suitable extrusion material filament that unwrapsfrom the spool to supply extrusion material to the extruder head 108.

The support arm 112 includes a support member and one or more actuatorsthat move the extruder head 108 during printing operations. In thesystem 100, one or more actuators 150 move the support arm 112 andextruder head 108 along the X and Y axes during the printing operation.For example, one of the actuators 150 moves the support arm 112 and theextruder head 108 along the Y axis while another actuator moves theextruder head 108 along the length of the support arm 112 to move alongthe X axis. In the system 100, the X/Y actuators 150 optionally move theextruder head 108 along both the X and Y axes simultaneously alongeither straight or curved paths. The controller 128 controls themovements of the extruder head 108 in both linear and curved paths thatenable the nozzles in the extruder head 108 to extrude thermoplasticmaterial onto the support member 102 or onto previously formed layers ofthe object 140. The controller 128 optionally moves the extruder head108 in a rasterized motion along the X axis or Y axis, but the X/Yactuators 150 can also move the extruder head 108 along arbitrary linearor curved paths in the X-Y plane.

The controller 128 is a digital logic device such as a microprocessor,microcontroller, field programmable gate array (FPGA), applicationspecific integrated circuit (ASIC) or any other digital logic that isconfigured to operate the printer 100. In the printer 100, thecontroller 128 is operatively connected to one or more actuators thatcontrol the movement of the support member 102 and the support arm 112.The controller 128 is also operatively connected to a memory 132. In theembodiment of the printer 100, the memory 132 includes volatile datastorage devices, such as random access memory (RAM) devices, andnon-volatile data storage devices such as solid-state data storagedevices, magnetic disks, optical disks, or any other suitable datastorage devices. The memory 132 stores programmed instruction data 134and three-dimensional (3D) object image data 136. The controller 128executes the stored program instructions 134 to operate the componentsin the printer 100 to form the three-dimensional printed object 140 andprint two-dimensional images on one or more surfaces of the object 140.The 3D object image data 136 includes, for example, a plurality oftwo-dimensional image data patterns that correspond to each layer ofthermoplastic material that the printer 100 forms during thethree-dimensional object printing process. The extruder head pathcontrol data 138 include a set of geometric data or actuator controlcommands that the controller 128 processes to control the path ofmovement of the extruder head 108 using the X/Y actuators 150 and tocontrol the orientation of the extruder head 108 using the Zθ actuator154. The controller 128 operates the actuators to move the extruder head108 above the support member 102 as noted above while the head extrudesthermoplastic material to form an object.

FIG. 1 depicts an additive manufacturing system 100′ that includes anactuator assembly 204 in the extruder head 108 that is operativelyconnected to the controller 128 that is operated to control the openingand closing of the valves for emitting thermoplastic material from theplurality of nozzles in the extruder head 108. Specifically, thecontroller 128 activates and deactivates different actuators in theassembly 204 connected to the valves in the extruder head 108 to emitthermoplastic material and form arrangements of the thermoplasticmaterial in each layer of the three-dimensional printed object 140.System 100′ also includes an extrusion material dispensing system 212that feeds filament from the supply 110 to the heater 208 at a rate thatmaintains the pressure of the thermoplastic material in the manifold 216within a predetermined range during operation of the system 100′. Thedispensing system 212 is one embodiment that is suitable for regulatingpressure of the thermoplastic material in the manifold. Additionally,the controller 128 is operatively connected to an actuator in thedispensing system 212 to control the rate at which the dispensing system212 delivers solid filament to a heater 208. The heater 208 softens ormelts extrusion material filament 220 fed to the heater 208 via driveroller 224. Actuator 240 drives the roller 224 and is operativelyconnected to the controller 128 so the controller can regulate the speedat which the actuator drives the roller 224. Another roller oppositeroller 224 is free-wheeling so it follows the rate of rotation at whichroller 224 is driven. While FIG. 1 depicts a feed system that uses anelectromechanical actuator and the driver roller 224 as a mechanicalmover to move the filament 220 into the heater 208, alternativeembodiments use one or more actuators to operate a mechanical mover inthe form of a rotating auger or screw. The auger or screw moves solidphase extrusion material in the form of extrusion material powder orpellets into the heater 208.

In the embodiment of FIG. 1, the heater 208 is formed from stainlesssteel and includes one or more heating elements 228, such aselectrically resistive heating elements, which are operatively connectedto the controller 128. Controller 128 is configured to connect theheating elements 228 to electrical current selectively to soften or meltthe filament of extrusion material 220 in channel 232 within the heater208. While FIG. 1 shows heater 208 receiving extrusion material in asolid phase as solid filament 220, in alternative embodiments, itreceives the extrusion material in solid phase as powdered or pelletizedextrusion material. Cooling fins 236 attenuate heat in the channel 232upstream from the heater 208. A portion of the extrusion material thatremains solid in the channel 232 at or near the cooling fins 236 forms aseal in the channel 232 that prevents thermoplastic material fromexiting the heater from any other opening than the connection to themanifold 216. The extruder head 108 can also include additional heatingelements to maintain an elevated temperature for the thermoplasticmaterial within the manifold 216. In some embodiments, a thermalinsulator covers portions of the exterior of the extruder head 108 tomaintain a temperature within the manifold 216.

To maintain a fluid pressure of the thermoplastic material within themanifold 216 within a predetermined range, avoid damage to the extrusionmaterial, and control the extrusion rate through the nozzles, a slipclutch 244 is operatively connected to the drive shaft of the actuator240. As used in this document, the term “slip clutch” refers to a deviceapplies frictional force to an object to move the object up to apredetermined set point. When the range about the predetermined setpoint for the frictional force is exceeded, the device slips so it nolonger applies the frictional force to the object. The slip clutchenables the force exerted on the filament 220 to remain constant nomatter how many valves are opened or how fast the actuator 240 drivesroller 224. This constant force can be maintained by either driving theactuator 240 at a speed that is higher than the fastest expectedrotational speed of the filament drive roller 224 or by putting anencoder wheel 248 on the roller 224 and sensing the rate of rotationwith a sensor 252.

The signal generated by the sensor 252 indicates the angular rotation ofthe roller 224 and the controller 128 receives this signal to identifythe speed of the roller 224. The controller 128 is further configured toadjust the signal provided to the actuator 240 to control the speed ofthe actuator. When the controller is configured to control the speed ofthe actuator 240, the controller 128 operates the actuator 240 so itsspeed is slightly faster than the rotation of the roller 224. Thisoperation ensures that the torque on the drive roller 224 is always afunction of the slip clutch torque. If one valve/nozzle combination isopen, the filament 220 moves slowly. If all of the actuator/valvecombinations in the assembly 204 are opened, the filament begins to movemore quickly and the controller 128 immediately operates the actuator240 to increase its speed to ensure that the output shaft of theactuator is turning faster than the speed of the roller 224 indicated bythe sensor 252. A delay inherently exists between the force applied tothe filament and the pressure of the thermoplastic material in thenozzle region of the extruder header. Empirical data of these delaysenable set points to be defined for the slip clutch that enable the slipclutch to be operated to provide more uniform pressure of thethermoplastic material in the nozzle region of the extruder head.

The signal generated by the sensor 252 can also be used by thecontroller 128 to regulate other aspects of the actuator 240. Forexample, the rate of feeding the filament to the heater of the additivemanufacturing system can be regulated to be proportional to the amountof thermoplastic material to be extruded by the extruder head. Bycontrolling the torque set point of the slip clutch, the controller canmaintain the proportional constant at 1 or slightly higher. Compensatingfor fluctuations outside of a predetermined range about the proportionalconstant of 1 is performed when clutch slips with reference to thetorque set point for the clutch in response to too much material beingfed to the heater and extruder head. This type of control can be used toregulate the rotation of the output shaft of the actuator.

In some embodiments, the torque level for the slip clutch 244 isempirically determined and set for operation of the system 100′. Inother embodiments, as noted above, a sensor 252 generates a signal thatidentifies the rotational speed of the roller 224 and the controller 128is further configured to use the signal from the sensor 252 to adjustthe signal from the controller that operates the actuator 240. In theembodiments that have this feedback loop, the feedback system can beconfigured as shown in FIG. 2 where the controller 128 implements aproportion-integral-derivative (PID) controller that adjusts theoperation of the actuator 240 with reference to the signal indicative ofthe rotational speed of the roller 224. This system is used to reducethe total number of cycles in which the slip clutch slips. As notedabove, the actuator 240 only needs to drive its output shaft at a ratethat is slightly faster than the rotational rate of the roller 224. Thistype of operation helps keep the slip clutch engaged so the number oftimes the clutch slips is minimized and the life of the clutch isextended.

The controller 128 has a set point stored in memory connected to thecontroller that identifies the slightly higher speed of the actuatoroutput shaft over the rotational speed of the roller 224. As used inthis document, the term “set point” means a parameter value that acontroller uses to operate components to keep the parametercorresponding to the set point within a predetermined range about theset point. For example, the controller 128 changes a signal thatoperates the actuator 240 to rotate the output shaft at a speedidentified by the output signal in a predetermined range about the setpoint. In addition to the commanded speed for the actuator, the numberof valves opened or closed in the actuator assembly 204 and the torqueset point for the clutch also affect the filament drive system 212operation. The resulting rotational speed of the roller 224 isidentified by the signal generated by the sensor 252. The PID controllerof controller 128 identifies an error from this signal with reference tothe differential set point stored in memory and adjusts the signaloutput by the controller to operate the actuator 240. Alternatively, thecontroller 128 can alter the torque level for the slip clutch or thecontroller 128 can both alter the torque level and adjust the signalwith which the controller operates the actuator.

An alternative embodiment of the feedback control system is shown inFIG. 3. In this embodiment, the controller 128 has been configured toimplement a proportion-integral-derivative (PID) controller that adjuststhe operation of the actuator 240 with reference to a signal indicativeof the pressure of the thermoplastic material in the manifold 216. Forthat purpose, a pressure sensor 260 (FIG. 1) is positioned to sense thepressure of the thermoplastic material in the manifold 216 and thesignal indicative of the pressure level generated by the sensor can beused by the PID controller of FIG. 2 to adjust the output signal foroperation of the actuator 240. This system is also used to reduce thetotal number of cycles in which the slip clutch slips. The controller128 has a set point stored in memory connected to the controller thatidentifies a pressure for the thermoplastic material in the manifold216. The controller 128 outputs a signal that operates the actuator 240to rotate the output shaft at a speed identified by the output signal.In addition to the commanded speed for the actuator, the number ofvalves opened or closed in the actuator assembly 204 and the torque setpoint for the clutch also affect the filament drive system 212operation. The pressure of the thermoplastic material in the manifold216 is identified by the signal generated by the sensor 260. The PIDcontroller of controller 128 identifies an error from this signal withreference to the pressure set point and adjusts the signal output by thecontroller to the actuator 240. Alternatively, the controller 128 canalter the torque level for the slip clutch or the controller 128 canboth alter the torque level and adjust the signal with which thecontroller operates the actuator.

The slip clutch 244 can be a fixed or adjustable torque friction discclutch, a magnetic particle clutch, a magnetic hysteresis clutch, aferro-fluid clutch, an air pressure clutch, or permanent magneticclutch. The clutch types that operate magnetically can have their torqueset points adjusted by applying a voltage to the clutches. This featureenables the torque set point on the clutch to be changed with referenceto print conditions. The term “print conditions” refers to parameters ofthe currently ongoing manufacturing operation that affect the amount ofthermoplastic material required in the manifold for adequate formationof the object. These print conditions include the type of extrusionmaterial being fed to the extruder head, the temperature of thethermoplastic material being emitted from the extruder head, the speedat which the extruder head is being moved in the X-Y plane, the positionof the feature being formed on the object, and the like.

In the embodiments having the pressure sensor 260, the controller 128can be further configured to monitor the pressure within the manifold216. If the pressure is greater than expected for the number of valvesopened at a particular time, the controller detects the condition as anindication of clogged nozzles. In the embodiments having the rotationalspeed sensor 252, the rotational rate of the roller is monitored and ifthe rate of rotation of the roller 224 is less than expected for thenumber of valves opened at a particular time, the controller 128 detectsthe condition as an indication of clogged nozzles. In either embodiment,the controller 128 can generate an alarm and take the system offline formaintenance. The extrusion feeding system 212 can also be operatedbefore any nozzles are opened to establish a predetermined pressure in amanifold in a predetermined range. Moreover, if the extruder head 108will not be operated for some significant period of time, the torquelevel for the slip clutch can be set to zero or near zero whiledepressurizing the extruder head.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems, applications or methods.Various presently unforeseen or unanticipated alternatives,modifications, variations or improvements may be subsequently made bythose skilled in the art that are also intended to be encompassed by thefollowing claims.

What is claimed:
 1. A method comprising: operating a mechanical mover toexert a force within a predetermined range to move extrusion materialinto a heater; operating the heater to melt the extrusion material andform thermoplastic material that moves into a manifold in an extruderhead; adjusting a torque set point for a slip clutch with reference toprint conditions; limiting with the slip clutch a force exerted by themechanical mover to the extrusion material; and operating an actuatorhaving an output shaft coupled to the mechanical mover defined by a pairof rollers that form a nip to rotate the rollers and move solid filamentinto the heater, the output shaft of the actuator rotates at a rategreater than the rate at which the mechanical mover moves the extrusionmaterial.
 2. The method of claim 1 further comprising: generating with asensor a signal indicative of a pressure of the thermoplastic materialwithin the extruder head; and adjusting with a controller rotation ofthe output shaft of the actuator with reference to the signal generatedby the sensor.
 3. The method of claim 1 further comprising: adjusting apressure set point for operating the actuator with reference to printconditions.
 4. The method of claim 3 wherein the print conditions areone or more of a type of extrusion material being moved to the heater, atemperature of the thermoplastic material, a speed at which the extruderhead is being moved, and a position of a feature being formed on theobject.
 5. The method of claim 1 wherein the operation of the slipclutch is operation of a torque friction disc clutch, a magneticparticle clutch, a magnetic hysteresis clutch, a ferro-fluid clutch, anair pressure clutch, or a permanent magnetic clutch.
 6. The method of 3further comprising: operating the actuator to produce a pressure of thethermoplastic material in the manifold in a predetermined range aboutthe adjusted pressure set point before the controller operates theextruder head to form an object with the thermoplastic material.
 7. Amethod comprising: operating a mechanical mover to exert a force withina predetermined range to move extrusion material into a heater:generating with a sensor a signal indicative of the rate at which themechanical mover rotates; operating an actuator with an output shaftcoupled to the mechanical mover; operating the heater to melt theextrusion material and form thermoplastic material that moves into amanifold in an extruder head; adjusting a torque set point for a slipclutch with reference to print conditions; limiting with the slip clutcha force exerted by the mechanical mover to the extrusion material;adjusting with a controller rotation of the output shaft of the actuatorwith reference to the signal generated by the sensor; operating with thecontroller at least one valve operatively connected between at least onenozzle in the extruder head and the manifold to extrude thermoplasticmaterial from the at least one nozzle selectively to form an object withthe thermoplastic material; detecting a clogging of the at least onenozzle with reference to a signal generated by a sensor that isindicative of a pressure in the manifold or a signal generated by asensor that is indicative of the rate at which the output shaft of theactuator rotates: and adjusting a rotation rate of the output shaft ofthe actuator or the torque set point of the slip clutch in response todetection of the clogging of the at least one nozzle.
 8. A methodcomprising: operating a mechanical mover to exert a force within apredetermined range to move extrusion material into a heater; operatingthe heater to melt the extrusion material and form thermoplasticmaterial that moves into a manifold within an extruder head; limitingwith a slip clutch a force exerted by the mechanical mover to theextrusion material; adjusting a torque set point of the slip clutch withreference to print conditions; operating with a controller at least onevalve operatively connected between at least one nozzle in the extruderhead and the manifold to extrude thermoplastic material from the atleast one nozzle selectively; operating an actuator coupled to themechanical mover to produce a pressure of the thermoplastic material inthe manifold in a predetermined range before the controller operates theat least one nozzle in the extruder head to form an object with thethermoplastic material extruded from the at least one nozzle; detectinga clogging of the at least one nozzle with reference to a signalgenerated by a sensor that is indicative of a pressure in the manifoldor a rotational speed of an output shaft of the actuator coupled to themechanical mover; and adjusting the torque set point of the slip clutchin response to detection of the clogging of the at least one nozzle. 9.The method of claim 8 further comprising: operating the actuator coupledto the mechanical mover at a speed that rotates the output shaft of theactuator at a rate greater than the rate at which the mechanical movermoves the extrusion material.
 10. The method of claim 9 furthercomprising: adjusting with a controller rotation of the output shaft ofthe actuator with reference to the signal indicative of the rotationalspeed of the output shaft of the actuator.
 11. The method of claim 9further comprising: adjusting with a controller rotation of the outputshaft of the actuator with reference to the signal indicative of thepressure within the manifold of the extruder head.
 12. The method ofclaim 8 wherein the print conditions are one or more of a type ofextrusion material being moved to the heater, a temperature of thethermoplastic material, a speed at which the extruder head is beingmoved, and a position of a feature being formed on an object beingformed with the thermoplastic material extruded from the extruder head.